Competitive considerations have caused metal part producers to develop processes for producing near net shape parts. These processes increase competitiveness by reducing scrap of high cost materials, lowering labor costs and minimizing subsequent processing or machining operations. These processes are also capable of producing large lots of intricate parts thereby further improving the overall economics.
Processes for producing near net shape parts include investment casting, powder metallurgy, semi-solid forming and metal injection molding. Most of these processes can process a wide range of metals including both ferrous and nonferrous metals. However, the range of metals that can be treated by metal injection molding has heretofore been predominantly limited to the processing of less reactive metal powders.
Powders of reactive metals, such as aluminum, magnesium and titanium, rapidly form surface oxide films which interfere with the production of parts having adequate green strength after molding, debinding or, in some instances, lower densities even after sintering. Unlike melting or semi-melting processes, which disperse or rupture the oxide film or powder metallurgy pressing operations, which employ sufficient pressures to provide good green strength, metal injection molding does not employ conditions which can readily mitigate the adverse effects of the oxide films present on the surfaces of most metal powders, particularly more reactive metal powders. Moreover, the oxide films on reactive metal powders are not readily reduced during debinding and/or sintering. For these reasons, metal injection molding has not been widely regarded as being useful for processing reactive metal powders.
Coated metal powders are well known in the art and have been used in conventional powder metallurgical processing. For example, electroless copper-coated aluminum has been powder metallurgically formed at pressures of four tons per square centimeter. See Japanese Patent Application No. 45,707, dated Aug. 28, 1973. Aluminum is electrolessly plated with copper employing an aqueous solution of copper sulfate, potassium sodium tartrate, sodium hydroxide and formaldehyde.
Aluminum powder has also been electrolessly plated with nickel by immersion in an aqueous solution of nickel chloride or sulfate, boric acid, sodium chloride and hydrofluoric acid. See USSR Patent No. 361,224, dated Nov. 16, 1970.
Another process for electroless coating aluminum powder with nickel is disclosed in an article by R. Narayw et al. in the International Journal of Powder Metallurgy and Powder Technology, Volume 19, April 1983, pages 101 to 105. After pretreatment to remove its oxide coating, aluminum powder is immersed in an aqueous bath of nickel chloride, sodium hypophosphite, sodium citrate and ammonium chloride.
The prior art also discloses other processes for coating metal powders including aluminum, titanium and magnesium.
Numerous U.S. patents disclose processes and binder systems for metal injection molding and some detail the problems associated with processing reactive metal powders. U.S. Pat. No. 4,964,907 to Kryata et al. discloses a process for metal injection molding titanium powder and examples of binder systems useful for metal injection molding. U.S. Pat. No. 4,765,950 to Johnson discloses and claims a novel binder system for metal injection molding which comprises two organic components one of which has a higher melting point that the other. The disclosures of U.S. Pat. Nos. 4,964,907 and 4,765,950 are incorporated herein by reference.